Recent years have seen the rapid development of CCD image sensors and their present widespread use in imaging systems for both amateur and professional applications. Their small size, electrical efficiency, cost effectiveness, etc., have made CCD image sensors the imaging units of choice not only for inexpensive consumer camcorders which provide image information that can be displayed on a television set, but for more critical uses where much higher picture resolution is needed, such as, for example, in digital color printing applications. Several types of CCD image sensors, and numerous variations thereof, are commercially available from various manufacturers. In one way, these types may be conveniently classified into two classes, namely a "high resolution" class of CCD image sensors and a "low resolution" or "television resolution" class.
A "high resolution" CCD image sensor, frequently also referred to as a "mega-pixel" CCD image sensor, has at least a total of one million picture cells ("pixels"), typically arranged in at least one thousand horizontal lines or rows (comprising in totality a "vertical" image picture frame) with each line or row containing at least one thousand pixels. For example, a high resolution CCD image sensor may have 1024 horizontal lines and 1024 cells (pixels) per line.
A "television resolution" or "low resolution" CCD image sensor also has its picture cells ("pixels") arranged in a number of horizontal lines or rows, with the number of lines approximating the number of horizontal lines required for a display which functions in accordance with a television standard. In the United States and a number of other countries that television standard, as established by the National Television Standards Committee (NTSC), calls for a complete vertical television display frame to have a total of 525 horizontal lines, made up of two "interlaced" fields ("odd" and "even" fields) of about 262.5 horizontal lines each. Thus, a "television resolution CCD image sensor typically has a number of horizontal pixel lines or rows which approaches or equals the 525 NTSC television standard horizontal lines. For example, a television resolution CCD image sensor may have 484 horizontal lines or rows of pixels (reasonably approximating the 525 NTSC lines), and each horizontal line of pixels may contain a total of 768 "active" (i.e., photo-sensitive) pixels.
Each one of the numerous design variations of the two classes of CCD image sensors, as classified above, is aimed at controlling the operation of a sensor in one particularly advantageous manner. For example, a high resolution CCD image sensor having as a design variation the inclusion of a so-called "electronic clock gate" (ECG array) and a single so-called "horizontal shift register" has to be controlled in its operation entirely differently compared to a high resolution CCD image sensor which has been designed to include two horizontal shift registers, and without an ECG array.
When it is desired to control a high resolution CCD image sensor (dedicated to be optimally controlled and operated in one particular manner in accordance with the one particular design) in an alternative mode, for example, in a "television resolution" mode, as compared to an inherent "high resolution mode," the level of complexity associated with providing appropriately timed and precisely related control signals to the sensor (for proper sensor function in the alternative mode) increases significantly. That increased level of complexity (for example, an entirely different assembly of electrical control circuits designed and dedicated for each one of the two desired modes of operation of the sensor) frequently completely negates the cost-effectiveness of the high resolution image sensor.
Thus, while it is possible, in principle, to operate an economically manufactured high resolution CCD image sensor not only in a high resolution mode for which it was inherently designed, but also in a low resolution mode (television resolution mode), it has not been possible heretofore to provide relatively economically a timing logic system for selectably controlling a high resolution CCD image sensor of the type having two horizontal shift registers (and without an ECG array) to provide a high resolution mode and alternatively a television resolution mode of picture imaging.
One particular apparatus and method for operating and controlling one particular high resolution CCD image sensor in a low resolution ("television resolution") mode of operation is disclosed in U.S. Pat. No. 5,264,939, issued on Nov. 23, 1993. That particular high resolution CCD image sensor (16), depicted in a FIG. 1 of the above patent, has two horizontal shift registers (26, 28) and an electronic clock gate (24). The image sensor (16) is controlled by several circuits (30; 32; 40; and 64) such that in the low resolution ("television resolution") mode of its operation the electronic clock gate (24) selectively "dumps" or discards certain rows of pixel information provided by the CCD image sensor (16), and rows of pixel information retained (i.e., not dumped) are shifted to only one horizontal shift register (26), and are shifted from there to a video display (42) to form an "interlaced" viewing signal in accordance with a television standard (NTSC-standard).
Alternatively, if it is desired to operate and control the particular high resolution CCD image sensor (16) in its high resolution mode, the circuits (30; 32; 40; and 64) are modified or adapted to provide control signals to the image sensor such that the electronic clock gate (24) is by-passed (i.e., made to be non-functional), and both horizontal shift registers (26; 28) are "activated" to receive and output therefrom all of the rows of pixel information of the image sensor (16).
Thus, U.S. Pat. No. 5,264,939 discloses the use of a number of discrete circuits (30; 32; 40; and 64) to generate an interlaced (i.e., television compatible) viewing signal at an output of a high resolution CCD image sensor of the type having an ECG array, and using only one of two horizontal shift registers in the generation of the viewing signal.
With respect to a "television resolution" CCD image sensor, such a sensor can, of course, be operated in a "television resolution" mode in which the sensor provides the requisite outputs of "odd" and "even" lines of pixels to form the "odd" and "even" interlaced display fields comprising the display frames on a standard television set. Alternatively, such a CCD image sensor may be controlled to output the signals from each row of pixels sequentially row-by-row.
However, a "television resolution" CCD image sensor fundamentally cannot provide a "high resolution" output.
The relatively complicated way of displaying television images in accordance with the NTSC standard is an outgrowth of the development of commercial broadcast television over the past fifty years to the present time. However, this way has served the test of time and is not easily departed from. A much more complete discussion of television (for black and white as well as color) together with the timing, blanking, synchronizing (sync) signals, etc. required by the NTSC "standard" is given in a book entitled Basic Television and Video Systems, by Bernard Grob, published by McGraw-Hill, Inc., Fifth Edition, 1984.
CCD image sensors are well known in the art, and will be briefly described hereinafter for a high resolution CCD image sensor of the type having two horizontal shift registers (line pixel registers). Such a CCD image sensor may have at the beginning of each horizontal line of cells a small number of cells (termed "Z ref" cells) used for determining a zero signal level. There are also a small number of cells (termed "D ref" cells) for determining a "dark" signal reference level, followed by a large number of "active" cells in the line for producing pixel image signals, and finally at the end of the line there are a few additional "Z ref" cells. One such high resolution CCD image sensor commercially available from the Eastman Kodak Co. (Part No. KAI-1000) has a total of 1032 cells in each horizontal line, with 2 "Z ref" cells at the beginning of the line, followed by 10 "D ref" cells, followed by 1014 "active" cells, followed by 6 "Z ref" cells at the end of the line, a total of 1032 cells. There are 1024 horizontal lines of these cells arranged in vertical columns.
As is well known, a television frequency sub-carrier signal (hereinafter termed "fsc") provides for the decoding and display in proper sequence of the color-components (e.g., red, green and blue) of standard television image signals. This is also explained in detail in the above-identified book by Bernard Grob. To synchronize the pixel image signals in each horizontal line of cells of a CCD image sensor with a television standard, the number of cells in a horizontal line is made a convenient multiple of the television frequency subcarrier ("fsc"). This will be explained in greater detail hereinafter. For the NTSC "standard", the "fsc" is 3.5795 MHz.
The synchronizing (sync) and control signals for a standard television system (e.g., NTSC) are well suited to the needs of video monitors (having lower resolution than the high resolution CCD image sensor is capable of providing) such as used in camcorder viewfinder displays. Generic television-standard timing generators specifically designed for producing these "standard" sync and control signals are commercially available off-the-shelf at low cost from a number of companies for use in conjunction with CCD image sensors designed inherently as "television resolution sensors." However, the standard sync and control signals produced by these commercially available timing generators are not directly usable as the vertical and horizontal control signals needed for a high resolution CCD image sensor of the type having two line pixel registers in either a high resolution mode or in a television resolution mode of operation.
As indicated previously, it is highly desirable to provide a simple, inexpensive and versatile timing logic system which incorporates a relatively inexpensive generic television-standard timing generator to optimally control the operation of a high resolution CCD image sensor of the type having two line pixel registers. The timing logic system should provide vertical and horizontal control signals for high resolution readout of the lines of video signals of the CCD image sensor from its two line pixel registers and, alternatively, control signals as needed for viewing in real time of video images in a television-standard display, these video images derived from the high resolution CCD image sensor of the type having two line pixel registers.